1. Technical Field
The invention relates to a process and apparatus for separation of petroleum coke on the basis of density, and particularly to producing a dense coke fraction used in the production of carbon anodes for aluminum production.
2. Description of the Prior Art
Carbon anodes are produced in a paste plant for the electrolytic reduction of aluminium. Roughly one tonne of carbon is consumed for every two tonnes of aluminium produced. A typical source of carbon for the paste plant is calcined petroleum coke. Coke density has an important impact on the anode properties. The coke raw material arrives in the plant as a material having a wide range of particle sizes and densities. In the anode plant, the coke is separated into various size fractions. A specific amount of each size fraction goes into the production of an anode in order to produce a predefined granulometry in the finished anode necessary to meet performance specifications.
WO 82/02503 by Forsberg et al., discloses an air stream separator for separating a heavier fraction such as stones, from a mixture of lighter particles with differing properties. The separator comprises a vibrating conveyor and a series of air nozzles directed in generally the conveying direction at the discharge end of the conveyor. Both lighter and heavier fractions are collected on a rocking chute having a horizontal portion and an inclined portion. The lighter fraction discharging from the conveyor is entrained in the air stream from the air nozzle onto the horizontal portion of the rocking chute, where the lighter fraction and is collected at a side outlet opening. WO 82/02503 teaches that the heavier stone fraction landing on the inclined portion of the rocking chute is not able to move upward and the heavier stone fraction is discharged at the lower inclined end of the separator.
U.S. Pat. No. 3,485,361 by Adams discloses a device for separating workpieces from machining chips and turnings. The device comprises a perforated grid on which a mixture of workpieces/chips/turnings are placed, the mixture is fed onto a shaker chute, at whose discharge the mixture is exposed to a high velocity air stream directed upwards through the grid at the mixture of workpieces/chips/turnings. The lighter chips and turnings are entrained in the air stream while the workpieces are appropriately discharged without being entrained.
U.S. Pat. No. 4,793,918 by Thomas discloses a gravity separator for particulate matter deposited on a perforated separator deck. The rate of separation is controlled by varying the end raise and the side tilt of the separator and includes and air supply directed to separate areas of the deck. The deck includes at least two perforated covers separated by transverse ribs, these features improve the solids separation. The U.S. Pat. No. 4,793,918 separator discloses that the heavier portion is discharged at the lower end of the tilted separator deck.
GB427,499 by Barker discloses an apparatus for separating dirt from coal where material enters an incoming fed trough and descends along a series of tilted reciprocating decks with an air-pervious bottom. The dirt/coal mixture is subjected to an upward current of air which helps to stratify the materials. The dirt/coal stratify while tumbling down the tilt of the decks. The denser and finer dirt is discharged at a first trough outlet with the lighter coal continuing down the tilted deck in a layer above the dirt being discharged. This lighter coal is subjected to second air-pervious deck similar to the first deck where a coal/dirt stream is discharged in a second trough outlet and returned to the incoming feed trough. The lightest and largest particle size clean coal fraction passes over a weir plate for collection.
U.S. Pat. No. 6,838,024 B1 relates to a production process to obtain carbon blocks intended for aluminium production. The process of U.S. Pat. No. 6,838,024 comprises separation of an initial carbon input into at least two size fractions, and taking most of these size fractions and crushing and mixing them together to obtain a product having a controlled proportion of ultra fine particles. This patent teaches that the presence of ultra fine particle grains increases the density of carbon blocks produced.
S. Wilkening teaches in “Potentialities in the Paste Plant”, Light Metals, 1997, pages 569-576, that coke may be separated on the basis of density. Wilkening states that given the same particle size recipe a higher apparent density will have a favourable effect on the strength permeability and resistivity of the carbon produced. An air knife is suggested by Wilkening for the coke density separation.